In recent years, development and study of magnetic tapes for recording computer data (so-called backup tapes) are energetically carried on. With the increase of throughput and the miniaturization of recording and reproducing systems, it is desired that magnetic tapes themselves for such use be miniaturized and have greater recording capacity as well. For the miniaturization of magnetic tapes, there are several means such as a means of fining of magnetic particles to be used, a means of increasing packing density, and a means of thinning a magnetic layer. Further, for the purpose of the achievement of rapid processing of a great deal of throughput in the system, magnetic tapes are strongly desired to have higher reliability more than before such that high running durability is ensured in repeating running at high speed, an error does not occur in use in broad environmental conditions (in particular, under fluctuating temperature and humidity conditions), and stable recording and reproducing can be carried out.
In the latest magnetic tapes, a magnetic recording medium having a two-layer structure comprising a nonmagnetic support having thereon a nonmagnetic layer and a thinner magnetic layer provided on the nonmagnetic layer is proposed (e.g., in JP-A-5-182178 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”.)) for solving the problem of thickness loss of the reduction of output and the like that occur in the monolayer constitution of a magnetic layer. High density recording becomes possible by the thinning of a magnetic layer, so that a higher recording capacity can be achieved.
For obtaining a great recording capacity, it is advantageous to use a magnetic tape comprising two-layer structure as above, further, thinning the thickness of a magnetic layer (the total thickness) is also a means that is often utilized for obtaining a great recording capacity. Since the strength of a magnetic tape per se generally becomes insufficient when the thickness of the magnetic tape is lessened, running durability is liable to lower, e.g., by the occurrence of deformation of the tape during running at high speed.
Accordingly, it is proposed to use support materials having relatively high rigidity, e.g., aramid, in a magnetic tape (JP-A-11-296839).
However, even in such a case, in the manufacturing process of a magnetic tape, when the web of a continuous length and broad wide magnetic tape is slit to a prescribed width (e.g., 3.8 mm, 8 mm, ½ inches), there are cases where the support protrudes from the magnetic layer and the back layer at the edge of the slit. Since a support is low in rigidity than a magnetic layer, the support sometimes deforms by sliding against the guide roller in the winding process after slitting. The deformation of a support is large on the side of sliding in contact with a guide roller, and small on the other side. Therefore, only one side of the support deforms in the state of being forced into the inside from the outside, and the thickness of the magnetic tape increases, so that the magnetic tape is not wound uniformly in the width direction and wound up in the state of one-side stretching.
The end face of winding in this state forms a radial pattern from the center of winding toward the outside. The radial pattern causes problems such as the failure in winding shape and output reduction. Further, whisker-like shavings (or thread-like shavings) are liable to occur in high speed running due to the deformation of support. The whisker-like shavings adhere to a drive as dirt, scatter in a cassette, adhere to the surface of a magnetic layer, cause clogging of a magnetic head, and often result in dropout.
JP-A-9-153212 proposes that a back layer should not protrude from the perpendicular line from the biggest convexity of a base so that a back layer is not shaved off. JP-A-11-296839 proposes that a back layer should be inside the perpendicular line from the biggest convexity of a base to prevent thread-like shavings from occurring.
JP-A-2000-207732 proposes that the outermost edge of a magnetic layer should not protrude from the outermost edge of a lower nonmagnetic layer for the improvement of durability.
However, prior techniques could not find yet the means capable of preventing the failure in winding shape and the reduction of output in a magnetic tape having high recording capacity, and the means effectively preventing the occurrence of whisker-like shavings (or thread-like shavings) at the time of high speed running.